OP-20-G

OP-20-G was the designation for a unit within the United States Navy’s communications intelligence organization during World War II. Located in Washington, D.C., it was primarily responsible for cryptanalysis of Japanese naval communications. OP-20-G was a key component of the United States' efforts to break Japanese codes and ciphers, most notably the Japanese Navy's main operational code, JN-25.

The organization was comprised of various sections specializing in different aspects of cryptanalysis, traffic analysis, translation, and dissemination of intelligence. Personnel included mathematicians, linguists, and specialists in naval operations. The information gleaned from OP-20-G's work was crucial to Allied victories in the Pacific Theater, including the Battle of Midway.

OP-20-G worked closely with other Allied codebreaking efforts, including those of the United States Army Signal Intelligence Service (SIS) and the British Government Code and Cypher School (GC&CS) at Bletchley Park. While collaboration existed, different organizations often focused on different aspects of Japanese communications or employed different methodologies.

After World War II, OP-20-G was reorganized and its functions absorbed into other naval intelligence entities. Its legacy remains significant, representing a vital element in the Allied intelligence effort during the war. The historical impact of OP-20-G highlights the importance of signals intelligence in modern warfare.